The present invention relates to intruder alarm systems.
In intruder alarm systems in which acoustic or other vibrations arising from the movements of an intruder within a protected area are sensed by one or more electro-mechanical transducers which provide electric signals from which an alarm condition may be recognised, it is necessary reliably to distinguish vibrations caused by a human intruder from those from other sources, such as rain or hail, small animals or nearby road, rail or air traffic, so as to avoid too many false alarms.
It is also necessary sometimes to avoid giving an alarm if vibrations resulting from human movements are detected but are found to be from outside the protected area.